In many applications it is desirable to amplify a signal of variable amplitude in a manner such that the level of the amplified signal is substantially constant. For this purpose, amplifiers with variable gains which are regulated according to a feedback mechanism are often used.
In U.S. Pat. No. 5805022 to Bruccoleri et al., a circuit is described in which the output of a variable gain differential amplifier drives a full-wave rectifier. The full wave rectifier output signals are used to drive a comparator. This comparator in turn drives a charge pump circuit whose output is converted from voltage-to-current to serve as a regulating signal for controlling the gain of the variable gain differential amplifier (VGA). When this circuit operates on an amplified input signal that is asymmetrical, Bruccoleri et al indicate that it has a disadvantage. Bruccoleri et al indicate that the portion of the asymmetrical signal emerging from the full wave rectifier with the greatest amplitude exclusively controls the gain of the VGA, which can be problematic in some instances.
In order to remedy this perceived shortcoming, Bruccoleri et al provide a gain-regulating circuit which substitutes two half-wave rectifiers for the full wave rectifier. Moreover, each half-wave rectified signal is supplied to its own comparator and charge-pump circuit. The result is a circuit that independently integrates the contribution of the output of each half-wave rectifier to establish an appropriate level of gain for the VGA.
Unfortunately, although this circuit operates well with both symmetrical and asymmetrical signals, the duplication of current sources, comparators and charge pumps required to implement this arrangement uses a large amount of silicon on a semiconductor device. This results in increased cost, size and power consumption and decreased reliability.
While the circuit arrangement of Bruccoleri et al. provides good performance, there is need for other circuit variations that can be used to reduce the amount of silicon consumed on a semiconductor device, for use when amplifying both symmetrical and non-symmetrical signals.